The present invention relates to a constant speed running apparatus which stores a running speed of a vehicle in memory and enables the running speed of the vehicle to be automatically maintained at the stored speed. More particularly, the present invention pertains to a constant speed running apparatus so designed that the level of a power source for driving the apparatus has already been raised to an operative level at the time of starting a constant speed running control.
Japanese Patent Publication No. 60-76429, published Apr. 30, 1985, employs a vacuum source such as an intake manifold as a power source for driving a constant speed running apparatus and which uses a vacuum pump as an auxiliary power source for insuring the vacuum obtained from the vacuum source. In particular, Diesel vehicles, which cannot utilize a vacuum from an intake manifold as a vacuum source for a constant speed running apparatus, have heretofore needed to be provided with a power source such as a vacuum pump for driving a constant speed running apparatus.
In general, a vacuum which is utilized for driving the constant speed running apparatus of the type described above is stored in a surge tank by the operation of a vacuum pump, and when the internal pressure in the surge tank rises above a set value as a result of consumption of the stored vacuum by a constant speed running control, a vacuum switch is turned on to deliver a detection signal to a control circuit which, in turn, activates the vacuum pump for a predetermined period of time so that the vacuum for driving the apparatus is restored in the surge tank.
The above described arrangement suffers, however, from the following problems.
Simultaneously with the starting of a constant speed running control, the vacuum stored in the surge tank is supplied to a vacuum actuator through piping, causing the internal pressure in the surge tank to become much higher than a set value for a vacuum switch, and thus resulting in undershoot, which means that the response of the vacuum actuator is unsatisfactory. The response of the vacuum actuator can be improved by increasing the capacity of the vacuum pump. However, an increase in the pump capacity undesirably involves various problems such as an increase in size of the vacuum pump, which leads to a rise in cost and weight. Further, since the operation of the vacuum actuator depends on the capacity of the vacuum pump, there may be variations in the operation of the vacuum actuator which involves problems in terms of reliability.
On the other hand, if the set value for the vacuum switch is lowered, the number of times of activation of the vacuum pump is increased and this disadvantageously leads to a reduction in the lifetime of components of the system, such as the vacuum pump, the vacuum switch, etc.